Conventionally, in natural gas mainly used as fuel, a small amount of an organic sulfur compound, such as t-butylmercaptan (TBM), tetrahydrothiophene (THT), and dimethylsulfide (DMS), is included as an odorant in order to make known the danger occurring upon leakage thereof. At present, natural gas, which is domestically supplied, is known to contain THT and TBM organic sulfur compounds mixed at a ratio of about 7:3, in which the total concentration of the odorant is about 4 ppm (15 mg/m3) (Korean Unexamined Patent Publication No. 2005-003351). Further, the odorant is contained in liquefied petroleum gas (LPG), the main component thereof includes DMS (dimethylsulfide) and TBM, and the total amount of the odorant is limited to about 30 ppm.
Although the organic sulfur compound odorant is required in the interest of preventing gas leakage accidents from becoming severe, in the case where natural gas or LPG is used as feed stock for hydrogen or synthesis gas, a poisoning phenomenon, which gradually decreases the activity of the catalyst during steam reforming, is caused. In the case of a typical steam reforming catalyst, the concentration of sulfur (S) allowable at an operation temperature of 700° C. is approximately 0.1 ppm (Catalysts Handbook 2nd Ed.). In practice, steam reforming for producing a reformed gas containing concentrated hydrogen through reaction between natural gas or LPG and steam is conducted using, as a reforming catalyst, a transition metal catalyst or a precious metal catalyst. As such, there are reports that these catalysts are easily poisoned by sulfur and sulfur compounds are formed on the catalyst surface thereof at concentrations as low as ppm or less (McCarty et al; J. Chem. Phys. Vol. 72, No. 12, 6332, 1980, J. Chem. Phys. Vol. 74, No. 10, 5877, 1981).
Thus, in the case where the fuel is used to prepare hydrogen or synthesis gas through steam reforming, the steam reforming catalyst is poisoned by sulfur, undesirably decreasing the activity of the catalyst. Accordingly, there is a need to perform desulfurization using a desulfurizing agent during steam reforming. Typical methods of removing the organic sulfur compound from hydrocarbon fuel include hydrodesulfurization. Hydrodesulfurization includes adding hydrogen to hydrocarbon fuel, decomposing an organic sulfur compound into hydrogen sulfide using a cobalt-molybdenum catalyst or nickel-molybdenum catalyst supported on alumina, and absorbing the produced hydrogen sulfide on a desulfurizing agent such as zinc oxide or iron oxide to thus remove it, as seen in Reactions 1 and 2 below.R—SH+H2→RH+H2S  Reaction 1H2S+ZnO→ZnS+H2O  Reaction 2
Through hydrodesulfurization and the subsequent absorption of hydrogen sulfide, the sulfur component contained in natural gas or LPG is known to decrease to about 0.1 ppm. However, because hydrodesulfurization and subsequent absorption are conducted at high temperatures of about 300˜450° C., a long period of time is taken to increase the temperature of the catalyst or absorbent, and furthermore, upon application to steam reforming, because some of the hydrogen prepared using a steam reformer is refluxed and then fed into a desulfurization reactor, the procedure thereof is complicated. Such operational problems may adversely affect hydrogen generators for residential fuel cell power generation systems or distributed fuel cell power generation systems, which require that devices be simple or that starting be rapid.
With the goal of removing sulfur from natural gas or LPG at lower temperatures without the use of hydrogen, by Osaka Gas, Japan, a copper-zinc desulfurization absorbent was developed through co-precipitation to thus apply it to the removal of thiophene at high temperatures (U.S. Pat. No. 6,042,798). However, this is disadvantageous because the temperature of the absorbent should be maintained at 200° C. or higher to assure a predetermined level or more of desulfurization efficiency.
TBM, THT, and DMS, which are odorants contained in natural gas or LPG, are known to be adsorbed on activated carbon or zeolite material at room temperature. Tokyo Gas, Japan, using an activated carbon fiber adsorbent having excellent adsorption desulfurization capability and an adsorbent resulting from ion-exchange of hydrophilic zeolite with one or two transition metals selected from among Ag, Fe, Cu, Ni, and Zn, removed a dimethyl sulfide (DMS) odorant from fuel gas (Japanese Unexamined Patent Publication Nos. 2003-20489 and 2004-99826). These desulfurization adsorbents are advantageous because different adsorbents having the ability to adsorb the sulfur compound are provided at the upper portion and the lower portion of a desulfurization reactor, or a mixture of two or more different adsorbents having the ability to adsorb the sulfur compound is provided, and thus it is possible to use them either at room temperature or at low temperatures. As one type among the desulfurization adsorbents, useful is an adsorbent in which a transition metal such as silver is supported on zeolite through ion exchange. Although an adsorbent, prepared by ion-exchanging or impregnating Na—Y zeolite with silver, exhibited DMS adsorption performance superior to activated carbon adsorbents when it contained 5 wt % or more of silver based on the total amount of the adsorbent, the TBM (t-butylmercaptan) adsorption performance thereof was similar to that of activated carbon only when the content of silver was increased to 18 wt % (Satokawa et al., Applied Catalysis B: Environmental, 56 (2005), p 51-56).
The desulfurization adsorbent may be regenerated by flowing steam, and adsorption and regeneration are repeated using two or more adsorption towers mounted in large-scale plants. However, in the case where miniaturization and lightweight are regarded as important, as in a residential fuel cell power generation system or a distributed fuel cell power generation system, it is difficult to provide a plurality of adsorption reactors to continuously conduct adsorption and regeneration. Thus, when comparing the activated carbon adsorbent with the silver-containing zeolite adsorbent from the point of views mentioned above, the activated carbon adsorbent has superior performance relative to the price thereof. However, in the case where only the activated carbon adsorbent is used, there is a problem in which the concentration of sulfur in gas flow discharged from the gas outlet of the adsorption reactor must be periodically analyzed using a predetermined gas analyzer in order to check the life span of the adsorbent. Further, the adsorption reactor should be replaced within a predetermined period usually before fully utilizing the adsorption capacity of the adsorbent. However, these problems may become more serious upon individual installation at independent places, like residential fuel cell power generation systems or distributed fuel cell power generation systems, unlike large-scale plants.